The invention relates to a control system and method for an articulated vehicle comprising a towing vehicle, a trailer and an autonomous emergency braking system. The invention can for example be applied in heavy-duty vehicles, such as trucks, buses and construction equipment.
An autonomous emergency braking (AEB) system, which may also be referred to as an advanced emergency braking system (AEBS), is an autonomous road vehicle safety system that monitors the traffic situation ahead of and that automatically apply the vehicle's brakes when certain conditions are fulfilled in order to avoid or mitigate accidents.
The current AEBS assumes a fixed brake-force and uses the same brake strategy regardless of road conditions, and the current AEBS could cause jack-knifing swing-out when the AEBS brakes an articulated vehicle comprising a towing vehicle and a trailer. Jack-knifing means the folding of the articulate vehicle such that it resembles the acute angle of a folding pocket knife. Swing-out, or trailer swing, is when the trailer skids to one side.
It is desirable to provide an improved control system and method for an articulated vehicle comprising an autonomous emergency braking system, which control system and method in particular may assure that the articulated vehicle can perform automated emergency braking without jack-knifing or swing-out during the AEBS braking.
According to the first aspect of the invention, there is provided a control system for an articulated vehicle comprising a towing vehicle, a trailer and an autonomous emergency braking system, wherein the control system comprises: brake control means adapted to apply a friction-estimating braking; brake force capacity estimation means adapted to estimate the brake force capacity of the vehicle as a function of longitudinal wheel slip based on the applied friction-estimating braking; axle load estimation means adapted to estimate the normal force on each wheel axle of the vehicle; friction estimation means adapted to estimate a friction coefficient based on the estimated brake force capacity and at least one of the estimated normal forces; and brake strategy adaptation means configured to adapt the brake strategy of the autonomous emergency braking system by adjusting the brake force for at least one wheel axle of the vehicle based on the estimated friction coefficient and the at least one wheel axle's estimated normal force.
The present invention is based on the understanding that by taking into account both current road friction and the normal force for each wheel axle, the brake force for one or more axle may be adjusted to avoid over or under usage of the vehicle's brakes, which may prevent jack-knifing or swing-out during AEBS intervention.
For preventing swing-out of the trailer, which for example may occur if the trailer is empty or lightly loaded, the brake strategy adaptation means may adapt the brake strategy such that over usage of the trailer's brakes is avoided. This may be achieved by reducing the brake force for each wheel axle of the trailer, in order to have some lateral tyre force “left” for the trailer. The brake force may be reduced compared to the original or default brake strategy.
For preventing jack-knifing of the articulated vehicle, the brake strategy adaptation means may adapt the brake strategy such that under usage of the trailer's brakes is avoided. This may be achieved, by increasing the brake force for each wheel axle of the trailer, for example such that (pushing) coupling forces between the towing vehicle and the trailer are reduced or eliminated. Alternatively or complementary, the brake force on the towing vehicle may be reduced. The brake force may be increased/reduced compared to the original or default brake strategy.
The trailer may comprise at least two heel axles, wherein the axle load estimation means is adapted to summarize the normal forces on the trailer's wheel axles, and wherein the brake strategy adaptation means is configured to adapt the brake strategy of the autonomous emergency braking system by adjusting the brake force for the trailer's wheel axles collectively based on the summarized normal forces. This is useful in case the it is not possible to brake the trailer axles individually. For a vehicle whose axes can be braked individually, the brake strategy adaptation means could individually adjust the brake force for each wheel axle.
A value representing the normal force on each wheel axle of the vehicle may be provided by an air bellow suspension system of the vehicle. The axle load estimation means may therefore be connected to, or form part of the air bellow suspension system.
The brake control means may be adapted to apply the friction-estimating braking directly when, or a predetermined time after, the autonomous emergency braking system has initiated a possible intervention. An advantage of this is that the actual road friction at the time and place for this particular intervention may be taken into account. Another advantage is that the friction-estimating braking may function as haptic warning that an autonomous emergency braking system intervention has started. The friction-estimating braking may also be part of a pre-brake phase of the autonomous emergency braking system. Furthermore, since the friction-estimating braking is applied only in conjunction with the autonomous emergency braking system intervention, there is no nuisance fuel consumption increase or brake-disc wear.
The brake control means may be adapted to apply the friction-estimating braking only to one wheel axle of the vehicle. An advantage of this is that the influence of the friction-estimating braking on the vehicle's normal behaviour and stability may be reduced.
The one wheel axle may be a pusher axle or a tare axle of the vehicle, not to lose stability of the vehicle. A pusher axle is a non-powered liftable axle placed immediately in front of drive axle, and a tag axle is a non-powered liftable axle placed behind a drive axle.
The one wheel axle (e.g. the pusher axle or tag axle) may be lifted when the friction-estimating braking is applied, to minimize the negative effect on vehicle stability caused by the friction-estimating braking. Furthermore, less braking force is needed to estimate the brake force capacity. What happens when the wheel is lifted is that the value of the braking force is scaled down which leads to less impact on the vehicle stability and that not high air pressure has to be used for the braking.
The brake force capacity estimation means may further be adapted to: find the optimal longitudinal braking point based on the estimated brake force capacity; and apply a slip margin to the optimal longitudinal braking point, which slip margin reduces the longitudinal braking force used by the autonomous emergency braking system. This to still have a tolerance, e.g. for negotiating a curve.
The slip margin may be set such that an anti-lock braking system (ABS) of the vehicle is not activated when the autonomous emergency braking system brakes the vehicle, and the longitudinal braking force used by the autonomous emergency braking system may be constant. An advantage of this is that the braking will be shorter than ABS active due to the relatively poor performance in today's ABS valves in commercial heavy duty vehicles.
Furthermore, the slip, margin may be increased if it is determined that the vehicle is under-steering, whereby the longitudinal braking force on the front wheel axle of the vehicle is further reduced.
The control system may further comprise an articulation angle measurement means adapted to measure the articulation angle between the towing vehicle and the trailer to detect swing-out or jack-knifing, wherein the brake strategy adaptation means is configured to release the braking of the trailer if swing-out is detected and to increase the braking of the trailer if jack-knifing is detected. This may farther improve the brake strategy.
According to a second aspect of the present invention, there is provided an articulated vehicle comprising a control system according to the first aspect.
According to a third aspect of the present invention, there is provided a control method for an articulated vehicle comprising a towing vehicle, a trailer and an autonomous emergency braking system, which method comprises the steps of: applying a friction-estimating braking; estimating brake force capacity of the vehicle as a function of longitudinal wheel slip based on the applied friction-estimating braking; estimating the normal force on each wheel axle of the vehicle; estimating a friction coefficient based on the estimated brake force capacity and at least one of the estimated normal forces; and adapting the brake strategy of the autonomous emergency braking system by adjusting the brake force for at least one wheel axle based on the estimated friction coefficient, and the at least one wheel axle's estimated normal force. This aspect may exhibit the same or similar features and/or technical effects as the other aspects of the invention.
According to a forth aspect of the present invention, there is provided a computer program comprising program code means for performing the steps of the control method of the third aspect when said program is run on a computer.
According to a fifth aspect of the present invention, there is provided a computer program comprising program code means for performing the steps of the control method of the third aspect when said program product is run on a computer.
Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.